In recent years, a technique for manufacturing a thin film transistor (hereinafter referred to as a TFT) over a substrate has made a great progress, and application to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility (also referred to as mobility simply) to a TFT formed using a conventional amorphous semiconductor film, and therefore high-speed operation has become possible. For this reason, it has been tried that a pixel, which was conventionally controlled by a driver circuit provided outside the substrate, is controlled by a driver circuit formed over the same substrate as the pixel.
A substrate used for a semiconductor device is expected to be a glass substrate rather than a quartz substrate or a single-crystal semiconductor substrate in terms of cost. However, a glass substrate is inferior in heat resistance and easy to be deformed due to heat. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over a glass substrate, laser annealing is often employed to crystallize a semiconductor film formed over the glass substrate in order to prevent the glass substrate from being deformed due to the heat.
Compared with another annealing method which uses radiant heat or conductive heat, the laser annealing has advantages that the process time can be shortened drastically and that a semiconductor substrate or a semiconductor film over a substrate can be heated selectively and locally so that thermal damage is hardly given to the substrate.
Laser oscillators used for the laser annealing are categorized as pulsed laser oscillators and continuous wave (CW) laser oscillators according to the oscillation method. In recent years, it has been known that the crystal grain formed in the semiconductor film becomes larger when using the CW laser oscillator such as an Ar laser or a YVO4 laser than when using the pulsed laser oscillator such as an excimer laser in crystallizing the semiconductor film. When the crystal grain in the semiconductor film becomes larger, the number of crystal grain boundaries in the channel region of a TFT formed using this semiconductor film decreases, and the carrier mobility becomes higher so that a more sophisticated device can be developed. For this reason, the CW laser oscillator is attracting attention.
Generally, a laser beam which is used for laser annealing of a semiconductor film has a linear spot shape and the laser annealing is conducted by scanning the linear spot of the laser beam on the semiconductor film. By shaping the laser beam into the linear spot, the area annealed by the laser beam at one time can be made larger. In this specification, laser beams having a linear shape and a rectangle shape on an irradiation surface are referred to as a linear beam and a rectangular beam, respectively. It is to be noted that the term of linear herein used does not mean a line in a strict sense but means a rectangle having a large aspect ratio (for example, aspect ratio of 10 or more (preferably 100 to 10000)). The laser beam is shaped into a linear spot because energy density required for sufficient annealing to an irradiation object can be secured. When sufficient annealing can be conducted to the irradiation object, the laser beam may be shaped into a rectangular or planar spot. In the future, laser annealing may be conducted with a planar beam.
On the other hand, when a silicon film having a thickness of several tens to several hundred nm, which is generally used in a semiconductor device, is crystallized with a YAG laser or a YVO4 laser, the second harmonic having a shorter wavelength than the fundamental wave is used. This is because the second harmonic has higher absorption coefficient of a laser beam to a semiconductor film than the fundamental wave and the semiconductor film can be crystallized effectively with the second harmonic. The fundamental wave is rarely used in this step.
However, laser annealing with the use of a CW laser oscillator has a problem in that an annealing state on an irradiation surface becomes inhomogeneous. The reason lies in that a laser beam emitted from a CW laser oscillator has Gaussian distribution in which the energy is attenuated from the center to the end. Therefore, homogeneous annealing is difficult.
The present applicant has already suggested a laser irradiation apparatus which solved the problem of the conventional laser irradiation apparatus.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-128421
According to the laser irradiation apparatus disclosed in Patent Document 1, two laser beams are used. Specifically, a CW laser beam converted into the second harmonic and a CW laser beam of the fundamental wave are delivered simultaneously.